This invention relates generally to audible warning devices such as automobile or boat horns, and other acoustic signaling devices. Usually these sounder devices are designed to be as small as possible and use as little electrical power as possible, but nevertheless must be capable of producing a high intensity acoustic output. There are many designs in prior art which have proven useful with some desirable features. These include the generation of perceived intense sound of more than one frequency occurring simultaneously, as discussed in U.S. Pat. No. 4,303,908 (Enemark), with the frequency differences being non harmonically related, as in U.S. Pat. Nos. 4,204,200 (Beyl) and 4,689,609 (Ko), or using combinations of arbitrary frequencies in order to make more alarming or raucous sounds. However, sounders with the above capabilities, and which also can produce very high acoustic peak pressure levels with very high efficiencies have not been optimally addressed by prior art.
In order to obtain high efficiencies many such devices use transducer elements of a piezoelectric nature as in U.S. Pat. Nos. 3,912,952 (Kumon) and 5,990,797 (Zlotchenko). These are usually resonant at some relatively high frequency, typically 2 to 4 kHz. However at these frequencies, the sound does not have the desired warning urgency characteristic that lower frequency sounders can produce.
A major problem of prior art is obtaining alarming lower frequency sounds while using highly efficient resonant transducers. This goal has been addressed in prior art with some success by using resonant transducers and frequency modulating their output to produce an effect similar to an emergency vehicle siren as in U.S. Pat. Nos. 4,088,995 (Paladino) and 4,195,284 (Hampshire), or by amplitude modulating their higher resonant frequency with a lower frequency as disclosed in U.S. Pat. No. 4,486,742 (Kudo). This produces a perceived sound of the lower frequency and the human ear seems to be mostly unaware of the higher carrier frequency. However, in prior art there seemed to be no highly efficient solution using this approach. This invention addresses this problem by optimizing the electrical drive going to the transducer to simultaneously best take advantage of the resonant nature of the transducer and yet deliver an audible and discernable lower set of frequencies for warning or signaling purposes.
A second problem is getting the sound to propagate primarily in a preferred direction and yet not require very large radiating surface dimensions, so that the directed acoustic energy of the source is concentrated in a defined angular range. It is well known that the angular radiation pattern of a sound source is controlled by the transverse dimensions of the source relative to the wavelength of the sound. In order to confine the radiation to a specific angle the transducer dimensions transverse to the propagation direction must be at least as large as about half the wave length of the sound being generated. So, for example, a directed sound of 300 Hz of wave length about 1 meter would require a radiator of about a half meter or more across. However a frequency of 3 kHz would only require a transducer of about 5 cm diameter. This invention attains the goal of lower frequency perception, but within the angular range set by the higher resonant frequency of a small, very efficient resonant transducer, or as will be described, by an array of such transducers.
A principal object of the invention is to provide a method for producing a very high intensity warning sound from an efficient and compact device which uses higher frequency resonant transducers.
Another object of the invention is to provide a warning device which has two or more frequencies which resemble conventional automobile horns or similar devices, but which utilize the high efficiencies of resonant transducers which have resonances at much higher frequencies.
A further object of the invention is to provide a method for controlling the direction of the radiated sound from a warning device consistent with the shorter wavelength of the higher resonant frequencies, while still producing sounds at the lower frequencies with longer wavelengths.
The first and second objectives are accomplished by the following approaches. As with prior or related art an electrical signal from an oscillator with a relatively low frequency, typically about 100 to 500 Hz, is amplified by a transistor means and then the voltage of this signal is increased by a step up transformer, with its output then applied to a piezo-electric sound transducer element. Typically the oscillator in prior art has been a simple xe2x80x9csquare wavexe2x80x9d digital logic level source, i.e. one which is on for the same duration as it is off. This then produces current flow through the transistor or other amplifier means to the transformer primary winding for 50% of the period of the oscillator. The transformer secondary winding then sends current to the piezoelectric element which is primarily a capacitive load element C. Because the transformer secondary is also an inductor with inductance L, it forms, with the capacitive transducer, a simple tank circuit with a natural frequency fRES given by:
fRES=(2*xcfx80*(L*C))xe2x88x921xe2x80x83xe2x80x83(1) 
which in this invention is preferably best tuned to the about the same frequency as the primary natural mechanical resonant frequency of the transducer, which in many cases is from about 2 to 4 kHz. In any case, whether or not so tuned, when the square wave signal from the transistor is switched from the off to the conducting or on state, the secondary winding and the transducer will electrically start to xe2x80x9cringxe2x80x9d or oscillate at the effective fundamental resonant frequency. However, at the moment when the transistor turns off, the transformer secondary voltage applied to the piezoelectric transducer abruptly reverses and the force on the transducer is reversed relative to when the voltage was first applied. The resulting effects on the amplitude of the motion of the transducer""s radiating element can vary widely, for if it comes at the wrong time, it can slow, stop, reverse, or decrease the amplitude of the motion and hence impair the acoustic output. If it occurs at the ideal time it will significantly enhance the output.
It is an important part of this invention to replace the xe2x80x9csquare wavexe2x80x9d oscillator with a rectangular pulse oscillator, i.e. one with a pulse with an independently adjustable xe2x80x9conxe2x80x9d duration, which is independent of the xe2x80x9coffxe2x80x9d duration and hence is independent of the oscillator frequency. The time that the pulse is xe2x80x9conxe2x80x9d, or the xe2x80x9con widthxe2x80x9d, is adjusted such that the end of the conducting period occurs at the time when the motion can best be increased in the direction it is already moving. This is analogous to first pushing a person in a swing, and then as the swing reverses direction pushing the swing back in the opposite direction to increase the range or amplitude of motion. So it is with the reverse voltage applied to the transducer. It must be timed optimally, or in other words, have the correct phase relationship to the already occurring oscillating transducer motion. The pulse generating oscillator is adjusted to have the optimum pulse xe2x80x9con widthxe2x80x9d which maximizes the motion of the transducer element. The benefit of this is to increase the peak acoustic amplitude output of the transducer.
An additional benefit is that in general the duration of the pulse is shorter than in the square wave case, so that less power is consumed. For example if the lower frequency is 300 Hz, with a period of 3.33 msec, and the resonant frequency is 2000 Hz, with a period of 0.50 msec, then the square wave pulse would have been on for one half of 3.33 msec or 1.67 msec, while in the improved case the xe2x80x9con widthxe2x80x9d of the pulse is best set on the order of about half of 0.50, or 0.25 msec. In such circuits which use a coupling transformer, the power supply current flow continues to increase during the on time of the transistor and so the power savings can be substantial.
A second part of this invention is to utilize more than one arbitrary lower frequency by adding, in a conventional logical xe2x80x9cORxe2x80x9d circuit, the output of two or more such oscillators, each with its own proper pulse width thus providing the optimum phase at turn off of each pulse to best enhance the motion of the transducer. This efficiently creates high output peak intensities but with the perceived sounds of the multiple lower frequencies. These multiple frequencies can be arbitrary, are not constrained by any harmonic relations nor are they dependent on the resonant frequency of the transducer. In fact, they may be chosen or tuned to be harmonically pleasing, or conversely may be discordant or comprised of some combinations which produce beat frequency effects that are advantageous for attention getting or even animal control purposes.
A third part of this invention uses the same principles as relied upon in the first two parts, but adds a plurality of pulses of the optimum pulse width from a resonant (higher) frequency pulse oscillator, gated on during the duration of the on time of the pulses from each of the lower frequency pulse oscillators. These multiple pulses are preferably timed to occur at the resonant frequency of the transducer, so that their phase relationship to the motion of the transducer is optimized. This last improvement is obtained in one embodiment by first mixing the lower frequency signals in a conventional digital xe2x80x9cORxe2x80x9d circuit and then, in a digital xe2x80x9cANDxe2x80x9d circuit, combining the xe2x80x9cORxe2x80x9d output signal with a pulse train from an oscillator of frequency near, or at, the resonant (higher) frequency of the transducer, with its on pulse widths optimized as previously described. In this case the resonant (higher) frequency pulse train is synchronously applied to the transducer in bursts with a burst duration of controlled length, and at the repetition rates of the lower frequencies. The main advantage of this configuration is an increased efficiency and higher peak output levels for a given size transducer. An added characteristic is that the spatial distribution of the radiated sound is governed by the higher frequency of the resonance and yet the perceived sound is at the lower frequencies. This last characteristic then is put to use in addressing the final objective of this disclosure as follows.
Finally, using the circuit last described, the synchronized signal with the proper phase can be sent to more than one transducer and these transducers can be arranged in various patterns, so that the angular distribution of the acoustic radiation will follow the well known laws for diffraction and interference, with the controlling wavelength corresponding to the higher resonant frequency, while the sounds will be heard at the lower frequencies. Surprisingly but understandably, the lower frequencies do not themselves have any appreciable effect on the radiation pattern. Thus the resonant frequency wavelength can be exploited to control the directivity of the transducer or that of an array of such transducers.
A final variation of this invention is to use multiple transducers with varying phase shifts or signal delays to different transducers, in order to electronically control the direction of the radiated lower pitch sound, but with the agility of the higher frequency and shorter wavelength corresponding to the transducer""s resonance.